Externally toothed gears, particularly those having fine pitch helical teeth for use in the transmissions of motor vehicles, are typically formed by cold extrusion. For example, U.S. Pat. No. 3,910,091 to Samanta, assigned to the assignee of the present invention, discloses a method and apparatus for cold extrusion of gears in which the first of two hollow billets or blanks is fed into an extrusion die having internal teeth. An advancing punch spears the first blank, and a suitable shoulder on the punch forces the blank partially through the extrusion die. The shoulder stops short of contacting the die teeth, and then returns to its home position. Thereafter, the second blank is inserted into the die and forced against the first blank, advancing the first blank totally through the die to produce full length teeth.
While the punch is stopped, the full-height unrelieved tooth land area of the die teeth minutely imprints the flanks and root fillet areas of the first blank, and because of its high unit loading usually leaves a characteristic stopping zone mark. While this stopping zone imperfection is relatively small, it is measurable and, if left untreated, can affect the performance of the system into which the gear is ultimately placed.
In the case of gears formed with helical teeth, the stopping zone mark can be burnished or ironed out by a normal post-extrusion tooth finishing operation such as rolling or shaving. However, spur gears having straight teeth are generally unrollable during post-extrusion finishing due to the absence of helical overlap. The stopping mark must, therefore, be allowed to remain, or an exorbitant volume of material must be removed from the trailing end of the blank through a facing operation. The stopping zone mark can also be of some concern on gears not intended for secondary finishing.